1. Field of the Invention
The present invention relates to a driving circuit for an LCD backlight lamp, more particularly to a driving circuit minimizing a leakage current due to stray capacitances residing in its lamp driving side.
2. Description of the Related Art
Generally, a battery is used as a power source for laptop computers that use an LCD as a main display device. An LCD uses a backlight lamp to supply an amount of light required for illuminating pixels to display data or information since it cannot generate light by itself. In addition, because a high voltage of about 1000-1500V is required to drive a backlight lamp, a low-voltage DC power supplied from a battery should be converted to high-voltage AC power. For satisfying this requirement, a driving circuit for a backlight lamp such as FIG. 1 is used.
FIG. 1 depicts a conventional driving circuit for an LCD backlight lamp. The driving circuit of FIG. 1 comprises a DC/DC converter 120 generating a DC voltage of higher level by switching the DC power supplied from a battery 110 according to a PWM (Pulse Width Modulation) control signal from a PWM controller 121; an inverter 130 consisting of an AC oscillator 131 which swings sinusoidally with an amplitude of the high DC voltage from the DC/DC converter 120 and a transformer T1 which boosts the AC output of the oscillator 131 to its secondary side; a Ballast capacitor C2 applying the boosted AC power from the transformer T1 to a backlight lamp 150 at initial state and absorbing some power to protect the driven lamp at stable state; a current sensor 160 sensing the current flowing in the lamp 150 after rectifying; and a luminosity controller 170 comparing the magnitude sensed by the current sensor 160 with an adjustable reference level which is set externally, and outputting a control signal to vary a duty ratio of the PWM control signal of the PWM controller 121 according to the comparison result.
The operation of the LCD backlight driving circuit configured as FIG. 1 will be explained in detail.
The DC/DC converter 120 always provides the inverter 130 with a high DC voltage by switching the DC power supplied from the battery 110 according to PWM control signal, and the inverter 130 converts the high DC voltage from the DC/DC converter 120 to high voltage AC power through the internal AC oscillator 131 and the transformer T1. While dissipating the supplied AC power, the lamp 150 emits light. At the moment when the lamp 150 starts to be driven, the Ballast capacitor C2 enables the high starting voltage (1000-1500V) to be instantly applied to the lamp 150, and then it absorbs some of the AC power outputted from the inverter 130 to protect the driven lamp 150, which guarantees stable operation of the lamp 150 after the lamp 150 is driven.
The current sensor 160 rectifies positive half waves through a diode D1 because the current driving the lamp 150 is an alternating current, and it flattens the rectified waves through a resister R7 and a capacitor C3. Then, the luminosity controller 170 compares the flattened magnitude outputted from the current sensor 160 with a reference which is adjustable manually, and outputs a difference signal, which is the result of the comparison, to change the duty ratio of the PWM-control signal of the PWM controller 121. Due to this feedback control based on a set reference and the fed back lamp current, it is possible to supply constant electric energy for the lamp 150, so that the desired brightness is maintained constantly.
In the conventional lamp driving circuit that operates as described above, the current flowing in the lamp 150 is fed back through the current sensor 160 and the luminosity controller 170 for PWM-control of the DC/DC converter 120 so that constant electric energy might be supplied to the lamp 150 to maintain a desired brightness. However, because the secondary side of the transformer T1 is connected to the primary side via a ground in order to establish a feedback loop as described above, stray capacitances are formed unwantedly along the high power path of the secondary side and around its windings and the lamp 150. Because of a leakage current induced by such stray capacitances, the efficiency of power consumption is lowered.
That is, in the conventional backlight driving circuit, stray capacitances Cx (marked as dotted lines) are formed, as depicted in FIG. 2, between the lamp 150 and a lamp protection reflector grounded, and along the high power path of the secondary side of the transformer T1. Therefore, a leakage current flows to a ground through the stray capacitances Cx. Because the leakage current due to the stray capacitances Cx is about 10% (in the condition of i=2Π fcv, f=50 kHz, V=700V, and C=about 20 pf) of the lamp driving current, all the energy provided from the secondary side of the transformer T1 is not used to drive the lamp 150, thus the efficiency of power consumption is not good.
It is an object of the present invention to provide an LCD backlight driving circuit being able to minimize a leakage current through stray capacitances by conducting feedback of some load current with electrical insulation between the primary and secondary side of a transformer.
A driving circuit for an LCD backlight according to the present invention comprises a DC/DC converter changing the level of an input DC power; an inverter converting the level-changed DC power into AC, boosting the converted AC power to higher voltage AC power, which is to be applied to a lamp, according to the ratio of a primary and a secondary winding; a feedback means sensing the AC current flowing in the lamp, feeding back the sensed current with electrical insulation between the primary and the secondary side, and flattening the fed back current; and a level controller comparing the flattened current with a reference signal, and providing the difference signal between the two compared signals to the DC/DC converter which adjusts a target level according to the difference signal. More particularly, the feedback means comprises a photo coupler rectifying the AC current flowing in the lamp and feeding back to the primary side, or an auxiliary transformer inducing the AC current of the lamp to its secondary winding with electrical insulation.
The driving circuit for an LCD backlight lamp according to the present invention, can eliminate stray capacitances which might reside in the lamp driving side, and minimize a leakage current through stray capacitances.